The present invention relates, in general, to a glow discharge tube for spectral-analytical tests, and more particularly to a system for improving the accuracy of conventional glow discharge tube systems.
German Pat. No. 1,589,389 and its Patent of Addition No. 1,910,461 disclose a glow discharge tube which is suitable for spectral-analytical tests. In the device disclosed therein, a tube is provided wherein a cathode glow light having a high luminous intensity is formed above the specimen, which is at the cathode potential. However, the glow light is limited by an annular anode support which extends to the specimen. The resulting annular gap between the end face of the anode support and the specimen probe forms a restricted path, or restrictor, between the anode, or main, cavity and the cathode cavity.
In the aforesaid German Patent and Patent of Addition the cathode cavity, or cathode space, of the glow discharge tube is evacuated until no glow discharge can arise therein. In the anode cavity, or anode space, a working pressure of a few millibars is established by means of a throttle valve fitted in the connecting line to the carrier gas source. With a voltage of approximately 1,000 to 1,500 v connected between the anode and the cathode, a glow discharge will be obtained with a current of about 100 to 200 mA. By maintaining the voltage, the current, or the power of the glow discharge tube at a constant value, perfectly linear and reproducible calibration curves are obtained for all the elements of the periodic system.
With conventional analytical concentrations in devices such as the foregoing, the margin of error in spectral-analytical tests lies distinctly below 1% relative to the specimen content. However, at the present time a number of applications, e.g., in the metallurgical industry, a still higher degree of accuracy is required.